HTML & CSS
I think this Firefox CSS is fantastic and I've used it on a number of different sites.
It allows multiple colours in the border.
It can add a really nice stylistic touch: https://developer.mozilla.org/en/CSS/-moz-border-top-colors
It's been a part of Firefox since version 1. It's a real shame that the Webkit crew have never seen it fit to be included with their build
Cute, though I think it's benefits will be somewhat restricted when all of the browsers enable CSS3's border-image event.